To accurately record radiation events in a controlled area, it is important for a detector and its associated apparatus to discriminate between radiation events caused by target particles (whose presence the apparatus expects to detect) and those caused by background radiation. In the case of Alpha detection a principal problem is that of Radon emission from the ground and surrounding building materials.
Where an industrial process requires testing for airborne contamination, it is common to employ a Continuous Air Monitor (CAM)
The purpose of a CAM is to detect airborne radioactive dust particles. A pump draws air at a known rate through a filter, thus trapping dust on the filter. A semiconductor detector faces the dust and monitors Alpha activity. Air must be free to flow onto and through the filter, so the detector is forced to measure Alpha activity across an air gap (typically ≈4 mm). Buildings emit Radon gas that decays to solid radioactive daughters which are trapped on the dust filter. It is typical for the total count due to the Radon daughter peaks to be ten or twenty times that of the required detection level of a release peak Thus, all Alpha-in-air monitoring must solve the problem of detecting small quantities of released material against a much larger and varying background.
Of the natural decay series, only two produce Radon gas and are therefore significant to CAMs.
Taking the 232Th series first:23290Th→22888Ra→22889Ac→22890Th→22488Ra→22086Rn→21684Po→21282Pb→21283Bi→64%→21284Po→20882Pb(stable) 36%→20881Tl→20882Pb(stable) 
The first solid daughter 216Po has such a short half-life (0.16 s) that it is unlikely to be trapped and detected on the filter card, so the Alpha CAM 232Th series starts with 212Bi which splits to decay via either 212Po (8.78 MeV) or 208Tl (6.08 MeV) to 208Pb.
The other significant series is 238U:23892U→23490Th→23491Pa→23492U→23090Th→22688Ra→22286Rn→21886Rn→21884Po→21482Pb→21483Bi→21484Po→21082Pb→21083Bi→21084Po→20682Pb(stable) 
The Alpha CAM 238U series begins with 218Po because it is the first solid daughter from the 222Rn gas, but 218Po emits 6 MeV Alphas that are almost indistinguishable from the 208Tl 6.08 MeV peak. Thus, the only clearly distinguishable Alpha seen from the 238U series is from 214Po (7.69 MeV). As far as a CAM is concerned, the series stops at 210Pb (22.3 year half-life), so the 5.3 MeV Alpha from 210Po is not seen.
Thus, an Alpha CAM must detect releases in the presence of three natural Radon decay daughter peaks at:                6.0/6.08 MeV        7.69 MeV        8.78 MeV        
Alphas leaving the filter are forced to travel across an air gap before reaching the detector and this causes each detected Alpha peak to acquire a low energy tail having a shape dominated by the geometry of the air gap, detector, and filter.
The three Radon Alpha peaks previously referred to are usually rejected by a curve fitting and subtraction process. A best-fit curve (usually a form of exponential enx) is fitted to each Radon peak, then this best-fit curve is subtracted from the measured spectrum. The process is usually carried out sequentially, starting with the highest Radon peak because the low energy tail of the 8.78 MeV peak still has significant counts below the 6.08 MeV peak and its tail.
The most accurate curve fit uses the “least squares” technique because this minimises chi squared, but the technique quickly becomes computationally intensive, which may have consequences for demonstrating system robustness in a safety case.
Because radiation is a statistical process, even a perfect subtraction must leave residual deviations about zero, even in the absence of any non-Radon activity. Because the subtraction causes some negative and some positive counts, a summation of the residual counts resulting from a perfect subtraction tends towards zero except where there has been a reading from an incident that is non-Radon dependent.
Although the Radon subtraction process described above is statistically valid, the assumption that any non-zero summation of the resulting residual can only be due to a non-Radon radioactive release may not necessarily valid. Investigated false alarms have been attributed (but not limited) to; xenon strobe lights, gamma rays, electro-magnetic interference, acoustic noise, abrupt changes in Radon concentration.
Release detection has two conflicting requirements:
Firstly, release detection should occur at an air activity that is As Low As Reasonably Practicable (ALARP) in order to minimise a worker's received dose.
Secondly, a false alarm could result in an entire building being evacuated and not being reoccupied until the area monitored by the instrument had been declared safe. False alarms are therefore expensive. Worse, false alarms diminish confidence in the system, possibly causing a true alarm to be ignored.
Release detection therefore has to strike a balance between the increasing probability of false alarms against the minimum detectable level.
Generally an organization's statutory Radiation Protection Adviser wants to maximise safety by driving alarm levels down, but facility managers must minimise lost time due to false alarms, which tends to drive alarm levels up. The two opposing forces therefore tend to an equilibrium whereby an alarm level is chosen that is as low as possible without causing an unacceptable number of false alarms—such a level is commonly known as ALARP. Unfortunately, this implies that the ALARP alarm level for each instrument must be found by trial and error in each facility. In practice, this level of detail is not practicable, and all instruments in a given facility are more likely to have their alarm levels determined by the single instrument that produces the most false alarms. This implies that all the other instruments have their alarm levels set too high.